Foamed polymeric materials are well known, and typically are produced by introducing a physical foaming agent into a molten polymeric stream, mixing the foaming agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions typically causes the foaming agent to gasify, thereby forming cells in the polymer. Under some conditions the cells can be made to remain isolated, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. As an alternative to a physical foaming agent, a chemical foaming agent can be used; which undergoes chemical decomposition (typically upon application of heat or pressure) in the polymer material causing formation of a gas.
Polymer melt must sustain high tensile stresses (or melt strength) without cell interface fracture. Cell interface fracture leads to neighboring cell coalescence and repeated fracture leads formation of very large cells and foam collapse. Once foam has formed, its geometry needs to remain stable while the thermoplastic cools and solidifies. Polyolefins such as low density polyethylene (LDPE) and polypropylene (PP) are commonly used in many non-crosslinked foam applications. For example, U.S. Pat. No. 6,583,193 discloses an extruded, coalesced polypropylene foam that is either open-celled which is useful for sound insulation applications or close celled which is useful for thermal insulation applications. Of these, polyethylene is preferred because of ease of foaming control. While foams including polypropylene components are known, in most cases such foams include significant proportion of additives that add controllability to the foaming process.
Polypropylene (PP) is relatively new to foam applications. Traditional PP's are semi-crystalline materials with linear molecular structures. Deficits in melt strength and extensional rheological properties limit its application to foams with a density higher than 500 kg/m3. This type of PP does not allow for controlling cell growth or preventing the cell wall from breaking during the foaming process.
To overcome the low tensile strength without sacrificing processability, blends of polyolefins with a component of high molecular weight (or low melt flow rate) material are commonly used. Polyolefin foams consisting of polyolefins of ultrahigh molecular weight, i.e., weight-average molecular weight from about 4×105 to 6×106 g/mol and higher are disclosed in U.S. Pat. No. 5,180,751. US '751 describes polypropylene foam made of polypropylene resins having a z-average molecular weight above 1×106 and a Mz/Mw ratio above 3.0. US '571 also states that unacceptable foam sheets show a unimodal molecular weight distribution, while resins which yield acceptable foam sheets show a bimodal molecular weight distribution.
PP can also be blended with a softer component such as copolymer of ethylene and vinyl acetate (EVA), ethylene-ethyl acrylate copolymer (EEA), ethylene-acrylic acid copolymer (EAA), or other ethylene copolymers having a low melting point to make soft foams for a variety of applications. For example, U.S. Pat. No. 6,590,006 discloses macrocellular foams comprising a blend of a high melt strength polypropylene and an ethylene copolymer such as EVA, EEA, and EAA for use in sound absorption and insulation applications.
U.S. Pat. No. 4,832,770 describes a method of manufacturing a foamed polypropylene resin from a mixture of 80 to 20 wt % of a crystalline polypropylene-ethylene block copolymer containing 20 wt % or less of ethylene and having a melt index of two or less and 20 to 80 wt % of a crystalline polypropylene-ethylene block or random copolymer containing 5 wt % or less of ethylene and having a melt index of 6 to 20 or a polypropylene homopolymer having a melt index of 6 to 20.
In addition to the above, foamed profiles of rubbers such as ethylene-propylene-diene (EPDM) rubber have been used in vulcanized form for high mechanical strength. The elastomeric characteristics of the EPDM rubber foam allow it to conform to the shapes needed and to be effectively compressed into gaps and corners of automotive openings when they are closed such that compressed foam hinders the entry of noise, dust and moisture. However, the construction of the EPDM rubber foam profiles and vulcanization of the EPDM requires careful and difficult handling.
Thermoplastic vulcanizate (TPV) compositions are thermoplastic with a pre-cross-linked rubber phases, e.g., EPDM rubber, and can be much more readily formed into complex shapes as with thermoplastic molding, retain mechanical strength much longer, and still provide resistance to moisture intake, as well as noise, dirt, etc. However, known TPV foam compositions tend not to provide the level of moisture intake prevention that the EPDM rubber foam compound counterparts do.
U.S. Pat. No. 6,713,520 describes thermoplastic vulcanizate foam compositions comprising a mixture that includes from about 15 to about 95 percent by weight of the rubber, and from about 5 to about 85 percent by weight of a thermoplastic component, based upon the total weight of the rubber and thermoplastic component combined, where the thermoplastic component includes from about 65 to about 90 percent by weight of a conventional thermoplastic resin and from about 10 to about 35 percent by weight of a random propylene copolymer based upon the total weight of the thermoplastic component.
WO 2004/016679A2 describes soft thermoplastic vulcanizate foams comprising polyolefin thermoplastic resin, an at least partially crosslinked olefinic elastomer, hydrogenated styrenic block copolymer, and optional additives. The soft foams have smooth surfaces, low water absorption, improved compression set and compression load deflection.
These compositions show better flexibility compared to that of the isotactic polypropylene alone, but are still lacking in other physical attributes. Physical blends also have the problems of inadequate miscibility. Immiscible components can phase separate or allow smaller components to migrate to the surface. Reactor blends, also called intimate blends (a composition comprising two or more polymers made in the same reactor or in a series of reactors), are often used to address these issues, however finding catalyst systems that will operate under the same environments to produce different polymers has been a significant challenge.
There is a strong and growing demand for polypropylene based foams in a market which has been traditionally served by materials such as polyurethane, polystyrene and polyethylene. Polypropylene based foams bring additional benefits to this market area, such as high heat resistance, excellent chemical resistance, and insulation properties. An ongoing need exists for polypropylene with good processability and high extensional rheological properties, which is desirable for foam applications.
Furthermore, a need exists for a relatively straightforward method of preparing polypropylene based material for foam application having desirable properties, particularly a method that does not require the use of a cross-linking agent, e.g., post-polymerization treatments, or the use of comonomers that have been found to result in undesirable gel formation, such as certain types of diene comonomers. According to the present invention there is provided foamed articles comprising a reactor polymer blend exhibiting a unique combination of a high melt flow rate combined with high tensile strength, and elongation at break.